The invention generally relates to the field of wireless data networks having several remote transceivers which share access to a centrally located base station transceiver. The base station is preferably located such that the signal path between it and the mobile units is minimally obstructed. Since the optimal location for the base station is often a remote location like a mountaintop, tall building or tower, it may be linked to a central office or dispatch center at another location by dedicated telephone lines. Such communication systems are typically used by public service agencies (police, fire, ambulance, etc.), as well as private businesses (taxicabs, couriers, utilities, etc.).
While wireless communication systems have been in use for many years, the demand for systems which improve upon the speed and reliability of digital data transmission from mobile units has greatly increased in recent years. This is due to the rapid implementation of data processing systems, i.e., computers, into virtually all types of systems, made possible, in part, by the decrease in size and power requirements for portable computers. Thus, it is now commonplace to find computers in mobile and handheld packages. Mobile data users find it particularly advantageous to link their data processing systems with other remote data processing systems without having to plug into a hardwired system such as the telephone network. For example, wireless linkages among mobile computer systems has widely expanded the utility of portable data processing in computer aided dispatch systems, automatic vehicle location systems and other applications where it is desirable to have a wireless connection for data transfer between mobile data terminals, mobile computer terminals, laptop computers, etc. A few specific examples include: 1) Automatic Number Identification (ANI) systems which enable public safety officers, taxi drivers and other mobile and portable radio users to identify themselves automatically and digitally with every radio transmission, thereby increasing security and management control; 2) automatic vehicle location (AVL) systems utilizing Global Positioning Systems (GPS) provide very accurate realtime positional information for each mobile unit of a fleet, a feature proving to be extremely valuable in total fleet management and Computer-Aided Dispatch (CAD) of mobile fleets; 3) digital messaging; and 4) access to large computer systems from remote locations for data entry and retrieval.
Since many users and potential users of these mobile data communications systems already have a large installed base of voice grade mobile communications systems, it is desirable and efficient to utilize the voice equipment for transmission of digital information in addition to voice. This is achieved by converting the data waveform to an audio waveform and passing it through the radio system in the same manner as voice is passed. Specifically, many mobile data communication systems use Frequency Modulation (FM) transmission techniques.
In a frequency modulation system, the signal to be transmitted modulates an RF carrier, thus converting the signal to be transmitted into a frequency deviation about a center frequency of an FM channel. The degree or amplitude of this frequency deviation must be calibrated and adjusted in each mobile unit. A common practical problem with FM mobile transceivers is that they are often installed by personnel who are not trained to test and/or adjust frequency deviation. Additionally, the deviation often tends to drift with time. If the deviation is improperly adjusted, the unit will behave somewhat erratically. The user of the mobile unit may not notice the poor performance in voice operation and has no way of detecting it when transmitting data. Thus, diagnosis and repair of improperly deviating transceivers is often time consuming and tricky. Additionally, when an improperly deviating unit is taken out of field service and tested in a repair facility, the degradation due to improper deviation may not be severe enough to cause the unit to fail. Diagnosis and correction of the problem requires a trained technician with specialized test equipment.
Deviation is also important from a regulatory point of view. Most every communication system which propagates signals through the air is provided a license by the Federal Communications Commission (F.C.C.) or some other governing body. Normally, part of the license specifies the amount of bandwidth which the licensed system is allowed to occupy as well as a maximum peak deviation. (See the F.C.C Code of Federal Regulations-Part 90.209) Since the occupied bandwidth of an FM signal increases with increasing deviation, excessive deviation can cause the licensed system to violate the terms of its license.
When a mobile modem is connected to a mobile radio, deviation of the audio signal is often set improperly. This happens because the modem is often used as a "bolt-on" accessory for existing radios, and must provide a wide range of adjustment. If the deviation is set too low, the result is a reduction in discriminator signal to noise ratio and a reduction in discriminator level. If deviation is set too high, the discriminator output becomes excessively large and often non-linear. Another result, as mentioned previously, is that the signal can exceed the legally allocated bandwidth of the system.
Improperly set deviation is logistically difficult and expensive to detect. When the normal system deviation for a system should be set at point A as shown in FIG. 3, the result of improper deviation is a reduction in signal to noise ratio which reduces the reliability of the data system. If the same driver uses the vehicle every day, he may never realize that his modem is not working as well as it should. Another problem often occurs if the driver notices that there is a problem and takes the vehicle into a repair facility. If the repair facility is closer to the base station than the driver's normal route, then the modem may work properly when tested and the technician may incorrectly conclude that the problem is operator error and makes no measurements or adjustments.
Many of the problems associated with wireless transmission of data utilizing voice quality FM transceivers could be reduced if frequency deviation errors could be detected and remedied in a timely manner. The present invention provides a means for both measuring deviation of the mobile units and correcting the output level of the base discriminator so that the base modem can decode the signal. The system also maintains a historical data base so that vehicles which present continuous deviation errors are automatically identified. Hence, the problem of detecting incorrect deviation is greatly mitigated.